1. Field of the Invention
The present invention relates to an ALC (Automatic Level Control) circuit for automatically adjusting the gain of an amplifier to establish an output signal level from the amplifier at a predetermined level.
2. Description of the Related Art
Many electronic cameras, such as a digital camera, a digital video camera, or the like, have a sound recording capability to record audio input through a microphone and a moving image recording capability to record moving images. Such an electronic camera having a sound recording capability generally includes a circuit for automatically adjusting the gain of an amplifier installed in a stage prior to a recording stage in accordance with a level of an input audio signal when the sound is recorded.
FIG. 1 shows an automatic level control circuit in a digital detection system using a programmable gain amplifier. In the automatic level control circuit of FIG. 1, an input audio signal amplified by a programmable gain amplifier 10 is converted from an analog signal into a digital signal by an analog/digital converter (an A/D converter) 20 installed in a stage after the programmable gain amplifier 10. Next, the level of the digital signal (an output signal level from the programmable gain amplifier 10) is detected by a level detector 30 and then compared with a predetermined reference level. Based on the obtained comparison result, the gain of the programmable gain amplifier 10 is adjusted according to a control signal output from a gain controller 40 to the programmable gain amplifier 10 such that the output audio signal from the programmable gain amplifier 10 (i.e. the input audio signal having been amplified by the programmable gain amplifier 10) is set to a predetermined audio signal level.
More specifically, when the level of the digital signal detected by the level detector 30 is greater than the predetermined reference level, an operation for stepping down the gain of the programmable gain amplifier 10 by one step (an attack action) is performed, whereas an operation for stepping up the gain of the programmable gain amplifier 10 by one step (a recovery action) is performed when the level of the digital output signal is smaller than the predetermined reference level. By repeating such adjustment to the gain achieved by the attack and recovery actions until the level of the digital signal reaches the predetermined reference level, the amplifier gain can be automatically controlled in accordance with the level of the input audio signal at the predetermined reference level, which enables the recording of audio input at a desired audio signal level.
The attack action performed in the automatic level control circuit is generally completed in a short time to minimize an adverse effect that an audio waveform loses its accurate shape due to an excessively large signal beyond a dynamic range. In contrast to this, the recovery action is completed over a relatively long time to prevent data concerning the sound intensity from being lost.
However, when a short-duration large signal, such as a signal of the sound of a window pane breaking, is sporadically input, problems as described below are introduced. With such a short duration signal, there is no necessity to maintain the amplifier gain in a reduced state, in contrast to a situation where a large signal continues for a significant amount of time. Accordingly, after the amplifier gain is rapidly reduced by the attack action invoked by the input of the short-duration large signal, it is necessary to restore the amplifier gain to its original state as quickly as possible. However, because in related art the recovery action is normally performed over a long time, amplifier gain which has been rapidly reduced is not promptly restored to the original state. As a result, an insufficient sound level continues for a certain time period until the time when the rapidly reduced amplifier gain is restored to the original state by the recovery action.
Although, in consideration of such insufficiency of the sound level, the operation time of the recovery action can be shortened, shortening the operation time of the recovery action results in a problematic deterioration in characteristics of a voice signal, including loss of the data on sound intensity, frequent occurrence of distortion in an audio waveform caused by frequent attack actions due to intermittent input of the large signal, and the like.